Chemokines are small, secreted pro-inflammatory proteins, which mediate directional migration of leukocytes from the blood to the site of injury. Depending on the position of the conserved cysteines characterizing this family of proteins, the chemokine family can be divided structurally into C, CC, CXC and CX3C chemokines that bind to a series of membrane receptors (Baggiolini M et al., 1997). These membrane receptors, all heptahelical G-protein coupled receptors, allow chemokines to exert their biological activity on the target cells, which may present specific combinations of receptors according to their state and/or type. The physiological effects of chemokines result from a complex and integrated system of concurrent interactions: the receptors often have overlapping ligand specificity, so that a single receptor can bind different chemokines. A single chemokine can bind to different receptors as well.
Studies on structure-activity relationships indicate that chemokines have two main sites of interaction with their receptors, the flexible amino-terminal region and the conformationally rigid loop that follows the second Cysteine. Chemokines are thought to dock onto receptors by means of the loop region, and this contact is believed to facilitate the binding of the amino-terminal region that results in receptor activation.
Usually, chemokines are produced at the site of injury and cause leukocyte migration and activation, playing a fundamental role in inflammatory, immune, homeostatic, hematopoietic, and angiogenic processes. Thus, these molecules are considered good target candidates for therapeutic intervention in diseases associated with such processes. The inhibition of chemokines, or of their receptors, can reduce leukocyte maturation, recruitment and activation, as well as other pathological processes related to angiogenesis or arteriosclerosis (Baggiolini M, 2001).
In addition to mutant inhibitory chemokines, antibodies and peptide and small molecule inhibitors blocking the receptors, the search for effective chemokine antagonists has also been extended to a series of viruses and other organisms that, when entering into contact with human or mammal hosts, show potent immunomodulatory activities affecting the host.
The viral mimicry of cytokines, chemokines, and their receptors may indicate strategies of immune modulation for developing therapeutic products. Recently, immunomodulatory factors expressed by haematophagous arthropods (such as mosquitoes, sandflies and ticks) have been reviewed (Gillespie, R D et al, 2001).
In particular, the salivary glands of ticks produce a complex mixture of bioactive molecules having, in particular, anti-inflammatory, anti-haemostatic and anti-immune activities. These include bioactive proteins that control histamine, bind immunoglobulins, or inhibit the alternative complement cascade or other proteases.
Despite the large amount of literature, only a few articles list cDNA sequences identified by random sequencing and differential screenings of libraries generated from various tick tissues and/or species. However, the large majority of these sequences have not been characterized biochemically or functionally, and many annotations are entered only on the basis of sequence similarity with known proteins involved in basic cellular functions, such as those previously characterised in tick salivary glands for enzymatic activities or inducing antibody response. In particular, there is no indication of tick proteins acting as CXC-chemokine binding proteins and functioning as CXC-chemokine antagonists.